Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.") It's part of what makes aviation
so exciting for all of us... just when you think you've seen it
all, along comes a scenario you've never imagined.
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators, and as
representatives of the flying community. Some of them, you may have
heard before... but for each of us, there will also be something we
might never have considered before, or something that didn't
"stick" the way it should have the first time we memorized it for
the practical test.
It is our unabashed goal that "Aero-Tips" will help our readers
become better, safer pilots -- as well as introducing our
ground-bound readers to the concepts and principles that keep those
strange aluminum-and-composite contraptions in the air... and allow
them to soar magnificently through it.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network. Suggestions for future Aero-Tips are
always welcome, as are additions or discussion of each day's tips.
Remember... when it comes to being better pilots, we're all in this
together.
Aero-Tips 06.06.06
We're all taught about Bernoulli
effect and lift generation as the primary means by which an airfoil
generates lift. But we often hear of another mechanism relating to
the reaction of air molecules striking the underside of an airfoil
with a positive angle of attack -- action and reaction, often
called the Newtonian theory of lift.
(Note: There's another theory called
the Magnus effect
we've already discussed in Aero-Tips).
NASA (the National Aeronautics and Space Administration) tells
us, however, that as a lift-generating theory, Newton is wrong. More correctly, the
idea that air hits the underside of the wing and rebounds away,
forcing the wing upward like a stone skipping on water, is
mistaken. Here's why:
- This theory concerns only the interaction of the lower surface
of the wing and the air. It assumes that all the wind-flow turning
(and therefore all the lift) is produced by the lower surface. But
the upper surface also turns the flow. In fact, when one considers
downwash produced by a lifting airfoil, the upper surface
contributes more flow turning than the lower surface. Newtonian
theory does not predict or explain this effect.
- Because this theory neglects molecules striking the upper wing
surface, it does not predict the negative lift present when the
angle of attack is negative. On the top of the airfoil, no vacuum
exists. Molecules are still in constant, random motion on the upper
surface (as well as the lower surface), and these molecules strike
the surface and impart momentum to the airfoil as well.
- The upper airfoil surface doesn't enter into Newtonian theory
at all. Using this theory, we would expect two airfoils with the
same lower surface but very different upper surfaces to give the
same lift. We know this doesn't occur in reality. In fact, spoilers
on the upper wing surface are used to change lift by disrupting
flow over the upper surface. Newtonian theory does not predict or
explain this effect.
- If we make lift predictions based on Newtonian theory, using
knowledge of air density and the number of molecules in a given
volume of air, the predictions are totally inaccurate when compared
to actual measurements. The chief problem with the theory is that
it neglects the physical properties of the fluid. Lift is created
by turning a moving fluid, the air, and all parts of the solid
object can deflect the fluid.
So is Newton wrong? No... but using Newton's laws of motion to
explain lift is. The interaction of air flow across the bottom and
the top of an airfoil, according to NASA, is the prime generator of
lift. Sounds like we're back to good ol' Bernoulli.
Aero-tips of the day: Understand basic
aerodynamics, so you can better understand the effect of changing
angle of attack.